1. Field of the Invention
The present invention relates to an asynchronous transfer mode (ATM) communication system.
In recent years, the spread of data communications has led to the use of public lines not only for conventional voice communications but also transfer of important data. Future communication networks, therefore, have to be able to transfer and exchange data with a higher quality. As a communication service network able to handle not only 64 Kb/s voice communications and low speed data, but also 150 Mb/s high speed data for moving images such as for television and high definition television images, attention is now being drawn by broadband integrated service digital networks (B-ISDN). These are now reaching the commercialization stage and are being standardized as to their interfaces. In a B-ISDN, unlike the conventional exchange method, use is made of ATM so as to enable equal handling of voice communications, low speed data, moving images, and other information of different speeds. That is, in an ATM switching network, information with different bandwidths are transferred and exchanged held in units of certain lengths called "cells", for the purpose of differentiation from conventional package communication.
2. Description of the Related Art
Such ATM includes variable bit rate (VBR) communication wherein cells (units of transfer of information in ATM) are produced and transferred each time information for communication is produced and constant bit rate (CBR) communication wherein cells are transferred periodically regardless of the existence of information as in the conventional communication systems.
An explanation will be made later, referring to the figures, of the concept of VBR communication, the concept of CBR communication, and the processing sequences, but here note that in the conventional processing sequence, when the communication bandwidth requested by a terminal equipment at the time of a call setup is larger than the allocable bandwidth of the ATM network, that communication cannot be received and the call is disconnected.
Therefore, communication cannot be performed until the allocable bandwidth at the ATM switching network side satisfies the requested bandwidth and the terminal equipment must repeatedly originate calls until it is allocable. That operation is not only forced on the terminal equipment side, but also results in meaningless processing performed on the ATM switching network side.
For the terminal equipment, there is the problem that when communication is not possible at the requested bandwidth, even if communication is desired at a lower communication quality for a while, that communication is not allowed.
Still further, in the state where communication has begun between two pieces of terminal equipment for which communication is allowed, when desiring to change the communication bandwidth due to a change in the traffic at the ATM switching network side, that change is not allowed and therefore there was the problem that efficient use of the communication lines and other resources on the ATM switching network side was not possible.